LOS ANGELES -- There is no enough evidence to show that large earthquakes like what happened in Japan would trigger nearby volcanic eruptions, says a US geophysicist.

After the devastating earthquake in Japan, people are worried that the earthquake may trigger the volcanic eruptions. Peter Cervelli, geophysicist at the US Geological Survey Volcano Science Center told Xinhua in an interview on Wednesday that he does not believe people have reason to be concerned.

"From time to time in the historic record we have observed a few cases where volcanic eruptions have followed large earthquakes. However, in most of those cases, it was a situation where the fault was large and directly under the volcano in question," said Cervelli.

He admitted that geophysicists still do not understand this relationship perfectly, it is a continuing scientific research.

"However, of all the possible consequences of this earthquake, the one that I will be most worried about, I would not include among them the chances of increased volcanic activity in the near future," said the geophysicist.

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In the case of Japan, Cervelli said: "So far we have seen no indication of any obvious triggering of volcanic activity."

He said there are some volcanoes in Japan that erupt very frequently, so if one of those happens to erupt at the same time or before and after the earthquake, it is really hard to say that volcano was triggered by the earthquake, because that volcano would erupt anyway.

He said there are a few examples in the historical record that suggest a connection between large earthquakes and volcanic eruptions, but even these, on close inspection, do not show a simple causative relationship.

Cervelli said the most unambiguous case of triggering is probably the November 29, 1975 magnitude 7.2 Kalapana, Hawaii earthquake, which was immediately followed by a small, and short- lived eruption at Kilauea volcano, Hawaii.

However, he said, in this case, the fault plane of the earthquake (i.e., the extent of the rupture) was directly beneath Kilauea Volcano. Also, at the time of the earthquake, Kilauea was showing signs of pressurization and was likely poised to erupt soon anyway.

According to Cervelli, Kilauea is one of the most active volcanoes in the world, and erupted frequently in the decades before and after the 1975 earthquake.

Another example of possible triggering occurred after the M-9.5 Chile earthquake on May 22, 1960, Cervelli said.

About 38 hours after the mainshock, Puyehue-Cordon Caulle in Central Chile erupted violently after being inactive for more than 25 years. In this instance, as at Kilauea above, it is likely that the fault rupture extended beneath the volcano, said the geophysicist.

According to Cervelli, establishing a statistical correlation between large earthquakes and subsequent volcanic eruptions requires a long and accurate record of both seismic and volcanic activity.

The global record of such activity is sparse, especially for events that occurred before 1900. While there are hints of connections in some cases, and a few convincing anecdotes, statistical analyses to date have shown only a weak correlation, said Cervelli.

And, even if a statistical correlation is shown, working to establish and explain a causal connection is another matter.

There is clear evidence that large, distant earthquakes can, and do, trigger swarms of small earthquakes in active hydrothermal systems, such as those in Yellowstone and Long Valley calderas. This phenomenon has been observed at many volcanoes following large earthquakes; the M-7.9 Denali Fault earthquake in Alaska is a good example, he said.

However, small earthquake swarms such as these are typically only recorded by sensitive seismic instrumentation and certainly do not fall into the category of an eruption.

According to Cervelli, conversely, volcanic activity is known to trigger earthquakes. For example, swarms of small earthquakes, rarely larger than M5, can accompany the upward movement of magma through the Earth's crust. Also, large volumes of magma that rise from deep within the lower crust to shallower depths are thought to perturb the stress field around a volcano, possibly triggering small earthquakes up to 25 km away.